When optical energy is transmitted through a diffuse medium, scattering causes redirection of the rays so that the geometric pathlength between the energy entrance point and the energy exit point no longer defines the distance energy travels within the specimen. In addition, substantial energy may be scattered back towards the entrance or otherwise away from the exit region where detection occurs, thereby reducing the detected signal. This signal is therefore variable depending on the scattering characteristics of the particular region of the specimen traversed by the optical radiation.
A further cause of interference is nonhomogeneous or layered distribution of specimen characteristics, e.g., the layers of skin and fat which cover muscle tissue, the skin which covers the flesh of a fruit or vegetable, or the coating of windows through which measurements are to be made. Often, it is desirable to eliminate the effects of the surface layers to provide information on the underlying portions of the specimen. The present invention is directed to solving these problems which cause inaccuracies in spectroscopic determination of qualitative or quantitative characteristics of the specimen.
An additional problem is the making of diffuse reflectance and transmittance measurements on small specimens. Present reflectance instruments are generally designed to illuminate the specimen and detect reflected energy over several square centimeters of area. It is sometimes necessary to work with small amounts of specimen, for example a single seed, which must be recovered intact for future use. The present invention also addresses both diffuse transmission and diffuse reflection measurements of small specimens.